A Drosophila dopamine 2-like receptor: Molecular characterization and identification of multiple alternatively spliced variants

Two naturally occurring mutations in the type 1 melanin-concentrating hormone receptor abolish agonist-induced signaling

The melanin-concentrating hormone (MCH) receptor type 1 (MCHR1) is a seven-transmembrane domain protein that modulates orexigenic activity of MCH, the corresponding endogenous peptide agonist. MCH antagonists are being explored as a potential treatment for obesity. In the current study, we examined the pharmacological impact of 11 naturally occurring mutations in the human MCHR1. Wild-type and mutant receptors were transiently expressed in human embryonic kidney 293 cells. MCHR1-mediated, Gα(i)-dependent signaling was monitored by using luciferase reporter gene assays. Two mutants, R210H and P377S, failed to respond to MCH. Five other variants showed significant alterations in MCH efficacy, ranging from 44 to 142% of the wild-type value. At each of the MCH-responsive mutants, agonist potency and inhibition by (S)-methyl 3-((3-(4-(3-acetamidophenyl)piperidin-1-yl)propyl)carbamoyl)-4-(3,4-difluorophenyl)-6-(methoxymethyl)-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (SNAP-7941), an established MCHR1 small-molecule antagonist, were similar to wild type. To explore the basis for inactivity of the R210H and P377S mutants, we examined expression levels of these receptors. Assessment by enzyme-linked immunosorbent assay revealed that cell surface expression of both nonfunctional receptors was comparable with wild type. Overnight treatment with SNAP-7941, followed by washout of antagonist, enhanced MCH induced signaling by the wild-type receptor and restored MCH responsiveness of the P377S but not the R210H variant. It is of note that the two loss-of-function mutants were identified in markedly underweight individuals, raising the possibility that a lean phenotype may be linked to deficient MCHR1 signaling. Formal association studies with larger cohorts are needed to explore the extent to which signaling-deficient MCHR1 variants influence the maintenance of body weight.

Dopamine down-regulates activity of alkaline phosphatase in Drosophila: the role of D2-like receptors

The effect of a rise in dopamine (DA) level as a result of a mutation, stress or pharmacological treatment on the activity of the enzyme of its synthesis, alkaline phosphatase (ALP) in females of Drosophila virilis and Drosophila melanogaster has been studied. It has been found that regardless of its nature, a rise in DA level has a negative effect on ALP activity, which indicates that DA down-regulates activity of the enzyme. The effects of bromocriptine (an agonist of Drosophila dopamine 2-like receptor (DD2R)) on ALP activity have been studied. ALP activity was found to drop in response to bromocriptine in flies. Conversely ALP activity was increased in flies with reduced DD2R expression (i.e. Actin5C-Gal4>UAS-ds-DD2R RNA-interference flies) vs. corresponding controls (i.e. Actin5C-Gal4>w1118 flies). Bromocriptine treatment of RNAi flies rescues ALP activity to the level typical of Actin5C-Gal4>w1118 flies. A change in DD2R number or availability was found not to prevent the response of ALP to heat stress, but to change the intensity of its response to the stress exposure. The role of D2-like receptors in down-regulation of ALP activity by DA and in ALP response to stressor in Drosophila is discussed.

The μ-opioid receptor variant N190K is unresponsive to peptide agonists yet can be rescued by small-molecule drugs

The μ-opioid receptor (MOR) plays an important role in modulating analgesia, feeding behavior, and a range of autonomic functions. In the current study, we investigated the degree to which 13 naturally occurring missense mutations affect the pharmacological properties of the human MOR. After expression of each receptor in human embryonic kidney 293 cells, signaling (Gα(i/o)-mediated) induced by peptide agonists was assessed using luciferase reporter gene assays. Multiple mutants (S66F, S147C, R260H, R265C, R265H, and S268P) show a significant reduction in agonist potency. At the N190K variant, agonist-mediated signaling was essentially absent. Enzyme-linked immunosorbent assay, microscopic analysis, and radioligand binding assays revealed that this mutant shows markedly reduced cell-surface expression, whereas all other receptor variants were expressed at normal levels. Surface expression of the N190K variant could be increased by incubation with the alkaloid agonist buprenorphine or with either naltrexone or naloxone, structurally related MOR antagonists. We were surprised to find that both putative antagonists, despite being inactive at the wild-type MOR, triggered a concentration-dependent increase in N190K receptor-mediated signaling. In contrast, peptidic ligands failed to promote expression or rescue function of the N190K mutant. Subsequent analysis of the N190K variant in an ethnically diverse cohort identified this isoform in a subgroup of African Americans. Taken together, our studies reveal that the N190K mutation leads to severe functional alterations and, in parallel, changes the response to established MOR ligands. The extent to which this mutation results in physiological abnormalities or affects drug sensitivity in selected populations (e.g., those with chronic pain or addiction) remains to be investigated.

Dopamine reveals neural circuit mechanisms of fly memory

A goal of memory research is to understand how changing the weight of specific synapses in neural circuits in the brain leads to an appropriate learned behavioral response. Finding the relevant synapses should allow investigators to probe the underlying physiological and molecular operations that encode memories and permit their retrieval. In this review I discuss recent work in Drosophila that implicates specific subsets of dopaminergic (DA) neurons in aversive reinforcement and appetitive motivation. The zonal architecture of these DA neurons is likely to reveal the functional organization of aversive and appetitive memory in the mushroom bodies. Combinations of fly DA neurons might code negative and positive value, consistent with a motivational systems role as proposed in mammals.

Modulation of motor behavior by dopamine and the D1-like dopamine receptor AmDOP2 in the honey bee

Determining the specific molecular pathways through which dopamine affects behavior has been complicated by the presence of multiple dopamine receptor subtypes that couple to different second messenger pathways. The observation of freely moving adult bees in an arena was used to investigate the role of dopamine signaling in regulating the behavior of the honey bee. Dopamine or the dopamine receptor antagonist flupenthixol was injected into the hemolymph of worker honey bees. Significant differences between treated and control bees were seen for all behaviors (walking, stopped, upside down, grooming, flying and fanning), and behavioral shifts were dependent on drug dosage and time after injection. To examine the role of dopamine signaling through a specific dopamine receptor in the brain, RNA interference was used to reduce expression levels of a D1-like receptor, AmDOP2. Injection of Amdop2 dsRNA into the mushroom bodies reduced the levels of Amdop2 mRNA and produced significant changes in the amount of time honey bees spent performing specific behaviors with reductions in time spent walking offset by increases in grooming or time spent stopped. Taken together these results establish that dopamine plays an important role in regulating motor behavior of the honey bee.

A pair of dopamine neurons target the D1-like dopamine receptor DopR in the central complex to promote ethanol-stimulated locomotion in Drosophila

Dopamine is a mediator of the stimulant properties of drugs of abuse, including ethanol, in mammals and in the fruit fly Drosophila. The neural substrates for the stimulant actions of ethanol in flies are not known. We show that a subset of dopamine neurons and their targets, through the action of the D1-like dopamine receptor DopR, promote locomotor activation in response to acute ethanol exposure. A bilateral pair of dopaminergic neurons in the fly brain mediates the enhanced locomotor activity induced by ethanol exposure, and promotes locomotion when directly activated. These neurons project to the central complex ellipsoid body, a structure implicated in regulating motor behaviors. Ellipsoid body neurons are required for ethanol-induced locomotor activity and they express DopR. Elimination of DopR blunts the locomotor activating effects of ethanol, and this behavior can be restored by selective expression of DopR in the ellipsoid body. These data tie the activity of defined dopamine neurons to D1-like DopR-expressing neurons to form a neural circuit that governs acute responding to ethanol.

D(2) receptors receive paracrine neurotransmission and are consistently targeted to a subset of synaptic structures in an identified neuron of the crustacean stomatogastric nervous system

Dopamine (DA) modulates motor systems in phyla as diverse as nematodes and arthropods up through chordates. A comparison of dopaminergic systems across a broad phylogenetic range should reveal shared organizing principles. The pyloric network, located in the stomatogastric ganglion (STG), is an important model for neuromodulation of motor networks. The effects of DA on this network have been well characterized at the circuit and cellular levels in the spiny lobster, Panulirus interruptus. Here we provide the first data about the physical organization of the DA signaling system in the STG and the function of D(2) receptors in pyloric neurons. Previous studies showed that DA altered intrinsic firing properties and synaptic output in the pyloric dilator (PD) neuron, in part by reducing calcium currents and increasing outward potassium currents. We performed single cell reverse transcriptase-polymerase chain reaction (RT-PCR) experiments to show that PD neurons exclusively expressed a type 2 (D(2alphaPan)) DA receptor. This was confirmed by using confocal microscopy in conjunction with immunohistochemistry (IHC) on STG whole-mount preparations containing dye-filled PD neurons. Immunogold electron microscopy showed that surface receptors were concentrated in fine neurites/terminal swellings and vesicle-laden varicosities in the synaptic neuropil. Double-label IHC experiments with tyrosine hydroxylase antiserum suggested that the D(2alphaPan) receptors received volume neurotransmissions. Receptors were further mapped onto three-dimensional models of PD neurons built from Neurolucida tracings of confocal stacks from the IHC experiments. The data showed that D(2alphaPan) receptors were selectively targeted to approximately 40% of synaptic structures in any given PD neuron, and were nonuniformly distributed among neurites.

Corazonin neurons function in sexually dimorphic circuitry that shape behavioral responses to stress in Drosophila

All organisms are confronted with dynamic environmental changes that challenge homeostasis, which is the operational definition of stress. Stress produces adaptive behavioral and physiological responses, which, in the Metazoa, are mediated through the actions of various hormones. Based on its associated phenotypes and its expression profiles, a candidate stress hormone in Drosophila is the corazonin neuropeptide. We evaluated the potential roles of corazonin in mediating stress-related changes in target behaviors and physiologies through genetic alteration of corazonin neuronal excitability. Ablation of corazonin neurons confers resistance to metabolic, osmotic, and oxidative stress, as measured by survival. Silencing and activation of corazonin neurons lead to differential lifespan under stress, and these effects showed a strong dependence on sex. Additionally, altered corazonin neuron physiology leads to fundamental differences in locomotor activity, and these effects were also sex-dependent. The dynamics of altered locomotor behavior accompanying stress was likewise altered in flies with altered corazonin neuronal function. We report that corazonin transcript expression is altered under starvation and osmotic stress, and that triglyceride and dopamine levels are equally impacted in corazonin neuronal alterations and these phenotypes similarly show significant sexual dimorphisms. Notably, these sexual dimorphisms map to corazonin neurons. These results underscore the importance of central peptidergic processing within the context of stress and place corazonin signaling as a critical feature of neuroendocrine events that shape stress responses and may underlie the inherent sexual dimorphic differences in stress responses.

The identification of inhibitors of Schistosoma mansoni miracidial transformation by incorporating a medium-throughput small-molecule screen

In Schistosoma mansoni, the miracidium-to-primary sporocyst transformation process is associated with many physiological, morphological, transcriptional and biochemical changes. In the present study, we use a medium-throughput small-molecule screen to identify chemical compounds inhibiting or delaying the in vitro transformation of miracidia to the sporocyst stage. The Sigma-Aldrich Library of Pharmacologically Active Compounds (LOPAC) contains 1280 well-characterized chemical compounds with various modes of action including enzyme inhibitors, antibiotics, cell-cycle regulators, apoptosis inducers and GPCR ligands. We identified 47 compounds that greatly reduce or delay this transformation process during a primary screen of live miracidia. The majority of compounds inhibiting larval transformation were from dopaminergic, serotonergic, ion channel and phosphorylation classes. Specifically, we found that dopamine D2-type antagonists, serotonin reuptake inhibitors, voltage-gated calcium channel antagonists and a PKC activator significantly reduced in vitro miracidial transformation rates. Many of the targets of these compounds regulate adenylyl cyclase activity, with the inhibition or activation of these targets resulting in increased cAMP levels in miracidia and concomitant blocking/delaying of larval transformation.

Two different forms of arousal in Drosophila are oppositely regulated by the dopamine D1 receptor ortholog DopR via distinct neural circuits

Arousal is fundamental to many behaviors, but whether it is unitary or whether there are different types of behavior-specific arousal has not been clear. In Drosophila, dopamine promotes sleep-wake arousal. However, there is conflicting evidence regarding its influence on environmentally stimulated arousal. Here we show that loss-of-function mutations in the D1 dopamine receptor DopR enhance repetitive startle-induced arousal while decreasing sleep-wake arousal (i.e., increasing sleep). These two types of arousal are also inversely influenced by cocaine, whose effects in each case are opposite to, and abrogated by, the DopR mutation. Selective restoration of DopR function in the central complex rescues the enhanced stimulated arousal but not the increased sleep phenotype of DopR mutants. These data provide evidence for at least two different forms of arousal, which are independently regulated by dopamine in opposite directions, via distinct neural circuits.

Functional differences between two CRF-related diuretic hormone receptors in Drosophila

In Drosophila, two related G-protein-coupled receptors are members of the corticotropin releasing factor (CRF) receptor subfamily. We have previously reported that one of these receptors, encoded by CG8422 is a functional receptor for a diuretic hormone, DH(44). Here, we report that the other CRF receptor subfamily member, encoded by CG12370, is also a receptor for the DH(44) neuropeptide. The lines of evidence to support this identification include increases in cAMP levels due to CG12370 receptor activation and the recruitment of beta-arrestin-GFP to the plasma membrane in response to DH(44) application. We compared these features of the receptors DH44-R2 (encoded by CG12370) and DH44-R1 (encoded by CG8422) and found fundamental differences in signaling, association with the arrestins, and peptide sensitivity. We found that the sensitivity of DH44-R2 to the DH(44) peptide is lower than that of DH44-R1, specifically an estimated EC(50) of 7.98E-07 moll(-1) for DH(44) by DH44-R2 to an EC(50) of 5.12E-09 moll(-1) by DH44-R1 and found that previous reports on the sensitivity of the tubule to DH(44) is in agreement with our measurements of DH44-R2 sensitivity. We employed a specific RNAi construct to selectively knock-down DH44-R2 expression and this led to heightened sensitivity to osmotic challenges. The functional characterization of this diuretic hormone receptor in Drosophila demonstrates a high degree of conservation of CRF-like signaling.

Investigation of a dopamine receptor in Schistosoma mansoni: functional studies and immunolocalization

A dopamine receptor (SmD2) was cloned from adult Schistosoma mansoni. The receptor has the classical heptahelical topology of class A (rhodopsin-like) G protein-coupled receptors (GPCR) and shares sequence homology with D2-like receptors from other species. The full length SmD2 cDNA was expressed in the yeast Saccharomyces cerevisiae and mammalian HEK293 cells. Functional assays in both expression systems revealed that SmD2 was responsive to dopamine in a dose-dependent manner, whereas other structurally related amines had no effect. Activation of SmD2 in mammalian cells caused an elevation in intracellular cAMP but not calcium, suggesting that the receptor coupled to Gs and the stimulation of adenylate cyclase. Pharmacological studies showed that the S. mansoni dopamine receptor was inhibited by apomorphine, a classical dopamine agonist, as well as known dopaminergic antagonists, including chlorpromazine, spiperone and haloperidol. SmD2 immunoreactivity was detected in membrane protein fractions of S. mansoni cercaria, in vitro transformed schistosomula and adult parasites, using a specific peptide antibody. When tested by confocal immunofluorescence, SmD2 was detected in the subtegumental somatic musculature and acetabulum of all larval stages tested. In the adults, SmD2 was enriched in the somatic muscles and, to a lesser extent, the muscular lining of the caecum. The results suggest that SmD2 is an important component of the neuromuscular system in schistosomes.

Comparative pharmacology of two D1-like dopamine receptors cloned from the silkworm Bombyx mori

Dopamine (DA) is a physiologically important biogenic amine in insect peripheral and nervous tissues.We recently cloned two DA receptors (BmDopR1 and BmDopR2) from the silkworm Bombyx mori and identified them as D1-like receptors, which activate adenylate cyclase to increase intracellular cAMP levels. In this study, these two receptors were stably expressed in HEK-293 cells, and the dose-responsiveness to DA and their pharmacological properties were examined using cAMP assays. BmDopR1 showed a dose-dependent increase in cAMP levels at DA concentrations up to 10(-7) M with EC(50) of 3.30 nM, while BmDopR2 required 10(-6) M DA for activation. In BmDopR1-expressing cells, DA at 10(-6)-10(-4) M induced 30-50% lower cAMP production than 10(-7) MDA. BmDopR2-expressing cells showed a standard sigmoidal dose-response, with maximum cAMP levels attained with 10(-5)-10(-4) M DA and EC(50) of 1.30 microM. Both receptors had similar agonist profiles, and the typical vertebrate D1-like receptor agonist SKF-38393 was ineffective. Experiments with antagonists revealed that BmDopR1 exhibits D1-like features. However, the pharmacology of BmDopR2 was distinct from D1-like receptors; the typical vertebrate D1-like receptor antagonist SCH-23390 was less potent than the nonselective antagonist flupenthixol and the D2-like receptor antagonist chlorpromazine. The rank order of activities of several antagonists for BmDopR1 and BmDopR2 was more similar to that of Drosophila melanogaster DA receptors than Apis mellifera DA receptors. These data suggest that DA receptors could be potential targets for specific insecticides or insectistatics.

Membrane-tethered ligands are effective probes for exploring class B1 G protein-coupled receptor function

Class B1 (secretin family) G protein-coupled receptors (GPCRs) modulate a wide range of physiological functions, including glucose homeostasis, feeding behavior, fat deposition, bone remodeling, and vascular contractility. Endogenous peptide ligands for these GPCRs are of intermediate length (27-44 aa) and include receptor affinity (C-terminal) as well as receptor activation (N-terminal) domains. We have developed a technology in which a peptide ligand tethered to the cell membrane selectively modulates corresponding class B1 GPCR-mediated signaling. The engineered cDNA constructs encode a single protein composed of (i) a transmembrane domain (TMD) with an intracellular C terminus, (ii) a poly(asparagine-glycine) linker extending from the TMD into the extracellular space, and (iii) a class B1 receptor ligand positioned at the N terminus. We demonstrate that membrane-tethered peptides, like corresponding soluble ligands, trigger dose-dependent receptor activation. The broad applicability of this approach is illustrated by experiments using tethered versions of 7 mammalian endogenous class B1 GPCR agonists. In parallel, we carried out mutational studies focused primarily on incretin ligands of the glucagon-like peptide-1 receptor. These experiments suggest that tethered ligand activity is conferred in large part by the N-terminal domain of the peptide hormone. Follow-up studies revealed that interconversion of tethered agonists and antagonists can be achieved with the introduction of selected point mutations. Such complementary receptor modulators provide important new tools for probing receptor structure-function relationships as well as for future studies aimed at dissecting the tissue-specific biological role of a GPCR in vivo (e.g., in the brain vs. in the periphery).

Interplay of JH, 20E and biogenic amines under normal and stress conditions and its effect on reproduction

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) are well known to play a gonadotropic role in adult insects. In Drosophila the mechanism of reciprocal regulation of JH and 20E is shown to be responsible for their proper balance. Dopamine is a mediator in this JH and 20E interplay. A proper balance between JH and 20E is crucial for the normal progress of oogenesis. An imbalance of gonadotropins leads to reproductive defects: a rise in JH titre leads to oviposition arrest, a rise in 20E level, to the degradation of vitellogenic oocytes. Upon a change in the level of one of the gonadotropins, the balance is restored owing to the relative change in the titre of the other.

Molecular cloning and characterization of cDNAs encoding dopamine receptor-1 and -2 from brain-suboesophageal ganglion of the silkworm, Bombyx mori

In order to better understand the relationship between dopamine and the release of diapause hormone into the blood, we cloned and characterized cDNAs encoding Bombyx mori dopamine receptor-1 and -2 (BmDopR1 and 2) from the pupal brain-suboesophageal ganglion. BmDopR1 and 2 had high similarities to group 1 (Drosophila melanogaster DOP1 and Apis mellifera DOP1) and group 2 (D. melanogaster DopR99B, A. mellifera DOP2 and Papilio xuthus DOP1), respectively. When BmDopR1 and 2 were expressed in human embryonic kidney (HEK) cells, they responded to dopamine by increasing intracellular cAMP levels, thus indicating the presence of D1-like receptors. There were no clear differences in BmDopR1 and 2 mRNA levels between brain-suboesophageal ganglion complexes of diapause and nondiapause egg producers during pupal-adult development. BmDopR1 and 2 mRNAs were concentrated in the mushroom body calyx rather than in the suboesophageal ganglion. Taking into account the results of earlier experiments on excised regions corresponding to mushroom bodies, BmDopR1 and 2 in the mushroom body apparently play a role in the release of diapause hormone.

Suppression of excitatory cholinergic synaptic transmission by Drosophila dopamine D1-like receptors

The physiological function of dopamine is mediated through its G-protein-coupled receptor family. In Drosophila, four dopamine receptors have been molecularly characterized so far. However, due largely to the absence of a suitable preparation, the role of Drosophila dopamine receptors in modulating central synaptic transmission has not been examined. The present study investigated mechanisms by which dopamine modulates excitatory cholinergic synaptic transmission in Drosophila using primary neuronal cultures. Whole-cell recordings demonstrated that cholinergic excitatory postsynaptic currents (EPSCs) were down-regulated by focally applied dopamine (10-500 microm). The vertebrate D1 specific agonists SKF38393 and 6-chloro-APB (10 microm) mimicked dopamine-mediated suppression of cholinergic synaptic transmission with higher potency. In contrast, the D2 agonists quinpirole and bromocriptine did not alter cholinergic EPSCs, demonstrating that dopamine-mediated suppression of cholinergic synaptic transmission is specifically through activation of Drosophila D1-like receptors. Biophysical analysis of miniature EPSCs indicated that cholinergic suppression by activation of D1-like receptors is presynaptic in origin. Dopamine modulation of cholinergic transmission is not mediated through the cAMP/protein kinase A signaling pathway as cholinergic suppression by dopamine occurred in the presence of the protein kinase A inhibitor H-89. In addition, an adenylate cyclase activator, forskolin, led to an increase, not a decrease, of cholinergic EPSC frequency. Finally, we showed that activation of D1-like receptors decreased the frequency of action potentials in cultured Drosophila neurons by inhibiting excitatory cholinergic transmission. All our data demonstrated that activation of D1-like receptors in Drosophila neurons negatively modulates excitatory cholinergic synaptic transmission and thus inhibits neuronal excitability.

Pharmacological analysis of human D1 AND D2 dopamine receptor missense variants

Drugs targeting dopamine receptors have been the focus of much research over the past 30 years, in large part because of their role in treating multiple pathological conditions including Parkinson's disease, schizophrenia, Tourette's syndrome, and hyperprolactinemia. Missense mutations in G protein-coupled receptors (GPCRs) can alter basal and/or ligand-induced signaling, which in turn can affect individuals' susceptibility to disease and/or response to therapeutics. To date, five coding variants in the human D1 receptor (hD1R; T37P, T37R, R50S, S199A, and A229T) and three in the human D2 receptor (hD2R; P310S, S311C, and T351A) have been reported in the NCBI single nucleotide polymorphism database. We utilized site-directed mutagenesis to generate cDNAs encoding these receptor isoforms. After expression in either HEK293 or neuronal GT1 cells, basal and ligand-induced signaling of each of these receptors was determined and compared to wild type. In addition, we investigated expression levels of each recombinant receptor and the effect of inverse agonist administration. Our data demonstrate that naturally occurring amino acid substitutions in the hD1R can lead to alterations in expression levels as well as in basal and ligand-induced signaling. The potency and efficacy of dopamine, synthetic agonists (i.e., fenoldopam, SKF-38393, SKF-82958, and SCH23390), and inverse agonists [i.e., flupenthixol and (+)butaclamol] were reduced at selected hD1R variants. Furthermore, inverse agonist induced effects on expression levels were sensitive to selected amino acid substitutions. In contrast to the hD1R variants, hD2R polymorphisms did not affect ligand function or receptor expression. The observation that the hD1R mutations induce significant alterations in pharmacologic properties may have implications both for disease susceptibility and/or therapeutic response to dopaminergic ligands.

A genome-wide inventory of neurohormone GPCRs in the red flour beetle Tribolium castaneum

Insect neurohormones (biogenic amines, neuropeptides, and protein hormones) and their G protein-coupled receptors (GPCRs) play a central role in the control of behavior, reproduction, development, feeding and many other physiological processes. The recent completion of several insect genome projects has enabled us to obtain a complete inventory of neurohormone GPCRs in these insects and, by a comparative genomics approach, to analyze the evolution of these proteins. The red flour beetle Tribolium castaneum is the latest addition to the list of insects with a sequenced genome and the first coleopteran (beetle) to be sequenced. Coleoptera is the largest insect order and about 30% of all animal species living on earth are coleopterans. Some coleopterans are severe agricultural pests, which is also true for T. castaneum, a global pest for stored grain and other dried commodities for human consumption. In addition, T. castaneum is a model for insect development. Here, we have investigated the presence of neurohormone GPCRs in Tribolium and compared them with those from the fruit fly Drosophila melanogaster (Diptera) and the honey bee Apis mellifera (Hymenoptera). We found 20 biogenic amine GPCRs in Tribolium (21 in Drosophila; 19 in the honey bee), 48 neuropeptide GPCRs (45 in Drosophila; 35 in the honey bee), and 4 protein hormone GPCRs (4 in Drosophila; 2 in the honey bee). Furthermore, we identified the likely ligands for 45 of these 72 Tribolium GPCRs. A highly interesting finding in Tribolium was the occurrence of a vasopressin GPCR and a vasopressin peptide. So far, the vasopressin/GPCR couple has not been detected in any other insect with a sequenced genome (D. melanogaster and six other Drosophila species, Anopheles gambiae, Aedes aegypti, Bombyx mori, and A. mellifera). Tribolium lives in very dry environments. Vasopressin in mammals is the major neurohormone steering water reabsorption in the kidneys. Its presence in Tribolium, therefore, might be related to the animal's need to effectively control water reabsorption. Other striking differences between Tribolium and the other two insects are the absence of the allatostatin-A, kinin, and corazonin neuropeptide/receptor couples and the duplications of other hormonal systems. Our survey of 340 million years of insect neurohormone GPCR evolution shows that neuropeptide/receptor couples can easily duplicate or disappear during insect evolution. It also shows that Drosophila is not a good representative of all insects, because several of the hormonal systems that we now find in Tribolium do not exist in Drosophila.

Effects of (-)-epigallocatechin gallate on the development of morphine-induced physical dependence

Among the various nervous systems and signaling components involved in the development of morphine withdrawal symptoms, sensitization of the brain dopaminergic nervous system and an increase in the cAMP levels in the locus coeruleus are believed to be the most important cellular events. This study tested the effects of (-)-epigallocatechin gallate (EGCG), a major compound of green tea, on the development of morphine-induced withdrawal symptoms. All the naloxone-precipitated withdrawal symptoms in morphine-dependent animals were inhibited by an EGCG pretreatment in a dose-dependent manner, being forepaw tremor, rearing, teeth chattering, urination, and wet dog shake were more sensitive than jumping and ptosis. In addition, EGCG showed moderate inhibitory effects on the morphine-induced increase in the cAMP levels in the locus coeruleus at 100 mg/kg and the signaling of the dopamine D2 receptor at 100 microM. Effects of EGCG on the sequestration of D2 receptor were inconclusive. These results suggest that EGCG has strong pharmacological activity against the development of morphine dependence, which can be partly explained by its inhibitory effects on the morphine-induced increase in the cAMP levels in the locus coeruleus and the signaling of the dopamine D2 receptor.

Roles of Protein Kinase C and Actin-Binding Protein 280 in the Regulation of Intracellular Trafficking of Dopamine D3 Receptor

D3 dopamine receptor (D3R) is expressed mainly in parts of the brain that control the emotional behaviors. It is believed that the improper regulation of D3R is involved in the etiology of schizophrenia. Desensitization of D3R is weakly associated with G protein-coupled receptor kinase (GRK)/β-arrestin-directed internalization. This suggests that there might be an alternative pathway that regulates D3R signaling. This report shows that D3R undergoes robust protein kinase C (PKC)-dependent sequestration that is accompanied by receptor phosphorylation and the desensitization of signaling. PKC-dependent D3R sequestration, which was enhanced by PKC-β or -δ, was dynamin dependent but independent of GRK, β-arrestin, or caveolin 1. Site-directed mutagenesis of all possible phosphorylation sites within the intracellular loops of D3R identified serine residues at positions 229 and 257 as the critical amino acids responsible for phorbol-12-myristate-13-acetate (PMA)-induced D3R phosphorylation, sequestration, and desensitization. In addition, the LxxY endocytosis motif, which is located between residues 252 and 255, was found to play accommodating roles for PMA-induced D3R sequestration. A continuous interaction with the actin-binding protein 280 (filamin A), which was previously known to interact with D3R, is required for PMA-induced D3R sequestration. In conclusion, the PKC-dependent but GRK-/β-arrestin-independent phosphorylation of D3R is the main pathway responsible for the sequestration and desensitization of D3R. Filamin A is essential for both the efficient signaling and sequestration of D3R.

Four Missense Mutations in the Ghrelin Receptor Result in Distinct Pharmacological Abnormalities

The growth hormone secretagogue receptor (GHSR) plays an important role in regulating food intake and energy homeostasis. In this study, we compared the pharmacological properties of four reported variants of the human GHSR (I134T, V160M, A204E, and F279L) with those of the wild-type receptor. Corresponding recombinant receptors were transiently expressed in either human embryonic kidney 293 or COS-7 cells. Basal as well as ligand-induced signaling was assessed by luciferase reporter gene assays and measurement of inositol phosphate production. In addition, receptor expression levels were monitored by whole-cell enzyme-linked immunosorbent assay. Ligand-independent signaling of the wild-type GHSR is significantly reduced with introduction of either the V160M or F279L substitutions, whereas basal activity of the A204E mutant is not detectable. Ghrelin potency is markedly increased at the V160M mutant, whereas the I134T variant is unresponsive to this endogenous agonist. In contrast, the I134T mutant responds to a known GHSR inverse agonist, [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P (SP-analog), albeit with reduced efficacy. Activity of the SP-analog at the V160M and F279L mutants is comparable to the wild type (WT) value. The overall expression level of each of the four GHSR variants is reduced relative to WT; however, the ratio between the intracellular and plasma membrane receptor density remains comparable. Treatment with the SP-analog significantly increases cell surface expression of each receptor with the exception of the A204E variant. Taken together, our studies reveal that naturally occurring GHSR mutations affect a wide range of pharmacologic properties. The physiological impact of these alterations within selected populations (e.g., obese, lean individuals) as well as the pharmacogenomic consequences of corresponding mutations remain to be further investigated.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Locomotor activity is regulated by D2-like receptors in Drosophila: an anatomic and functional analysis

In mammals, dopamine 2-like receptors are expressed in distinct pathways within the central nervous system, as well as in peripheral tissues. Selected neuronal D2-like receptors play a critical role in modulating locomotor activity and, as such, represent an important therapeutic target (e.g. in Parkinson's disease). Previous studies have established that proteins required for dopamine (DA) neurotransmission are highly conserved between mammals and the fruit fly Drosophila melanogaster. These include a fly dopamine 2-like receptor (DD2R; Hearn et al. PNAS 2002 99(22):14554) that has structural and pharmacologic similarity to the human D2-like (D2R). In the current study, we define the spatial expression pattern of DD2R, and functionally characterize flies with reduced DD2 receptor levels. We show that DD2R is expressed in the larval and adult nervous systems, in cell groups that include the Ap-let cohort of peptidergic neurons, as well as in peripheral tissues including the gut and Malpighian tubules. To examine DD2R function in vivo, we generated RNA-interference (RNAi) flies with reduced DD2R expression. Behavioral analysis revealed that these flies show significantly decreased locomotor activity, similar to the phenotype observed in mammals with reduced D2R expression. The fly RNAi phenotype can be rescued by administration of the DD2R synthetic agonist bromocriptine, indicating specificity for the RNAi effect. These results suggest Drosophila as a useful system for future studies aimed at identifying modifiers of dopaminergic signaling/locomotor function.

Eleven new putative aminergic G-protein coupled receptors from Amphioxus (Branchiostoma floridae): identification, sequence analysis and phylogenetic relationship

We have identified eleven novel aminergic-like G-protein coupled receptor (GPCRs) sequences (named AmphiAmR1-11) by searching the genomic trace sequence database for the amphioxus species, Branchiostoma floridae. They share many of the structural motifs that have been used to characterize vertebrate and invertebrate aminergic GPCRs. A preliminary classification of these receptors has been carried out using both BLAST and Hidden Markov Model analyses. The amphioxus genome appears to express a number of D1-like dopamine receptor sequences, including one related to insect dopamine receptors. It also expresses a number of receptors that resemble invertebrate octopamine/tyramine receptors and others that resemble vertebrate alpha-adrenergic receptors. Amphioxus also expresses receptors that resemble vertebrate histamine receptors. Several of the novel receptor sequences have been identified in amphioxus cDNA libraries from a number of tissues.

A review of neurohormone GPCRs present in the fruitfly Drosophila melanogaster and the honey bee Apis mellifera

G protein-coupled receptor (GPCR) genes are large gene families in every animal, sometimes making up to 1-2% of the animal's genome. Of all insect GPCRs, the neurohormone (neuropeptide, protein hormone, biogenic amine) GPCRs are especially important, because they, together with their ligands, occupy a high hierarchic position in the physiology of insects and steer crucial processes such as development, reproduction, and behavior. In this paper, we give a review of our current knowledge on Drosophila melanogaster GPCRs and use this information to annotate the neurohormone GPCR genes present in the recently sequenced genome from the honey bee Apis mellifera. We found 35 neuropeptide receptor genes in the honey bee (44 in Drosophila) and two genes, coding for leucine-rich repeats-containing protein hormone GPCRs (4 in Drosophila). In addition, the honey bee has 19 biogenic amine receptor genes (21 in Drosophila). The larger numbers of neurohormone receptors in Drosophila are probably due to gene duplications that occurred during recent evolution of the fly. Our analyses also yielded the likely ligands for 40 of the 56 honey bee neurohormone GPCRs identified in this study. In addition, we made some interesting observations on neurohormone GPCR evolution and the evolution and co-evolution of their ligands. For neuropeptide and protein hormone GPCRs, there appears to be a general co-evolution between receptors and their ligands. This is in contrast to biogenic amine GPCRs, where evolutionarily unrelated GPCRs often bind to the same biogenic amine, suggesting frequent ligand exchanges ("ligand hops") during GPCR evolution.

Identification of amino acid determinants of dopamine 2 receptor synthetic agonist function

The human dopamine 2 receptor (hD2R) modulates locomotor activity, hormone secretion, and neuropsychiatric function. Current knowledge of the hD2R structure is in large part derived from mutagenesis studies and molecular pharmacologic analysis together with homology modeling using bovine rhodopsin as a template. In this study, we utilized comparison of the Drosophila D2-like receptor (DD2R) with the hD2R as a novel approach for identifying candidate amino acids that are determinants of ligand potency and/or efficacy. We focused our studies on four dopaminergic ligands that are used in the treatment of Parkinson's disease: bromocriptine, pergolide, piribedil, and ropinirole. All four ligands are potent agonists at the wild-type hD2R, whereas only bromocriptine shows comparable function at the DD2R. We performed site-directed mutagenesis to replace hD2R amino acids (modeled to project into the ligand binding pocket) with corresponding fly residues, and vice versa. Substitution of three amino acids in the hD2R with the homologous DD2R residues (V91A, C118S, and L170I) led to a pronounced loss of pergolide potency and efficacy. A converse triple amino acid substitution of human residues into the fly receptor (DD2R-A133V/S160C/I211L) markedly enhanced pergolide efficacy and potency at the mutant DD2R. The same substitutions also converted piribedil and ropinirole, which lacked appreciable activity on the DD2R, to partial agonists. These findings show the important role of these three residues in drug-receptor interactions. Our study illustrates that comparison of a mammalian receptor with an invertebrate homolog complements previously described strategies for defining G protein-coupled receptor structure-function relationships.

Arthropod D2 receptors positively couple with cAMP through the Gi/o protein family

The pyloric network is an important model system for understanding neuromodulation of rhythmic motor behaviors like breathing or walking. Dopamine (DA) differentially modulates neurons within the pyloric network. However, while the electrophysiological actions of DA have been well characterized, nothing is known about the signaling events that mediate its effects. We have begun a molecular characterization of DA receptors (DARs) in this invertebrate system. Here, we describe the cloning and characterization of the lobster D(2) receptor, D(2 alpha Pan). We found that when expressed in HEK cells, the D(2 alpha Pan) receptor is activated by DA, but not other monoamines endogenous to the lobster nervous system. This receptor positively couples with cAMP through multiple Gi/o proteins via two discrete pathways: 1) a G alpha mediated inhibition of adenylyl cyclase (AC), leading to a decrease in cAMP and 2) a G beta gamma-mediated activation of phospholipase C beta (PLC beta), leading to an increase in cAMP. Alternate splicing alters the potency and efficacy of the receptor, but does not affect monoamine specificity. Finally, we show that arthropod D(2) receptor coupling with cAMP varies with the cellular milieu.

Identification and characterisation of the dopamine receptor II from the cat flea Ctenocephalides felis (CfDopRII)

G protein-coupled receptors (GPCRs) represent a protein family with a wide range of functions. Approximately 30% of human drug targets are GPCRs, illustrating their pharmaceutical relevance. In contrast, the knowledge about invertebrate GPCRs is limited and is mainly restricted to model organisms like Drosophila melanogaster and Caenorhabditis elegans. Especially in ectoparasites like ticks and fleas, only few GPCRs are characterised. From the cat flea Ctenocephalides felis, a relevant parasite of cats and dogs, no GPCRs are known so far. Thus, we performed a bioinformatic analysis of available insect GPCR sequences from the honeybee Apis mellifera, the mosquito Anopheles gambiae, the fruit fly Drosophila melanogaster and genomic sequences from insect species. Aim of this analysis was the identification of highly conserved GPCRs in order to clone orthologs of these candidates from Ctenocephalides felis. It was found that the dopamine receptor family revealed highest conservation levels and thus was chosen for further characterisation. In this work, the identification, full-length cloning and functional expression of the first GPCR from Ctenocephalides felis, the dopamine receptor II (CfDopRII), are described.

Point mutations in either subunit of the GABAB receptor confer constitutive activity to the heterodimer

The GABA receptor (GABABR) is a class C G protein-coupled receptor (GPCR) that functions as an obligate heterodimer, composed of two heptahelical subunits, GABABR subunit 1 (R1) and GABABR subunit 2 (R2). In this study, we generated and pharmacologically characterized constitutively active GABABR mutants as novel tools to explore the molecular mechanisms underlying receptor function. A single amino acid substitution, T290K, in the R1 agonist binding domain results in ligand-independent signaling when this mutant subunit is coexpressed with wild-type R2. Introduction of a Y690V mutation in the putative G protein-coupling domain of R2 is sufficient to confer moderate constitutive activity when this subunit is expressed alone. Activity of the Y690V mutant can be markedly enhanced with coexpression of wild-type R1. Coexpression of both mutant subunits (R1-T290K and R2-Y690K) leads to a further increase in basal signaling. Potencies of the full agonists R-(+)-beta-(aminomethyl)-4-chlorobenzenepropanoic acid hydrochloride (baclofen) and GABA are increased at the constitutively active versus the corresponding wild-type receptors. The mutant GABABR variants provided a sensitive probe enabling detection of inverse or partial agonist activity of molecules previously considered neutral antagonists. Our studies using constitutively active isoforms provide independent support for a model of GABABR function that takes into account 1) ligand binding by R1, 2) signal transduction by R2, and 3) modulation of R2-induced function by R1. Furthermore, we demonstrate that certain hallmark features of constitutive activity as originally established with class A GPCRs (e.g., enhanced agonist potency and affinity), are more generally applicable, as suggested by our finding with a class C heterodimeric receptor.

Widely distributed Drosophila G-protein-coupled receptor (CG7887) is activated by endogenous tachykinin-related peptides

Neuropeptides related to vertebrate tachykinins have been identified in Drosophila. Two Drosophila G-protein-coupled receptors (GPCRs), designated NKD (CG6515) and DTKR (CG7887), cloned earlier, display sequence similarities to mammalian tachykinin receptors. However, they were not characterized with the endogenous Drosophila tachykinins (DTKs). The present study characterizes one of these receptors, DTKR. We determined that HEK-293 cells transfected with DTKR displayed dose-dependent increases in both intracellular calcium and cyclic AMP levels in response to the different DTK peptides. DTK peptides also induced internalization of DTKR-green fluorescent protein (GFP) fusion constructs in HEK-293 cells. We generated specific antireceptor antisera and showed that DTKR is widely distributed in the adult brain and more scarcely in the larval CNS. The distribution of the receptor in brain neuropils corresponds well with the distribution of its ligands, the DTKs. Our findings suggest that DTKR is a DTK receptor in Drosophila and that this ligand-receptor system plays multiple functional roles.

Molecular cloning and characterization of crustacean type-one dopamine receptors: D1alphaPan and D1betaPan

Dopamine (DA) differentially modulates identified neurons in the crustacean stomatogastric nervous system (STNS). While the electrophysiological actions of DA have been well characterized, little is known about the dopaminergic transduction cascades operating in this system. As a first step toward illuminating the molecular underpinnings of dopaminergic signal transduction in the crustacean STNS, we have cloned and characterized two type-one DA receptors (DARs) from the spiny lobster (Panulirus interruptus): D(1alphaPan) and D(1betaPan). We found that the structure and function of these arthropod DARs are well conserved across species. Using a heterologous expression system, we determined that DA, but not serotonin, octopamine, tyramine or histamine activates these receptors. When stably expressed in HEK cells, the D(1alphaPan) receptor couples with Gs, and DA elicits an increase in [cAMP]. The D(1betaPan) receptor responds to DA with a net increase in [cAMP] that is mediated by Gs and Gz.

A human glucagon-like peptide-1 receptor polymorphism results in reduced agonist responsiveness

Glucagon-like peptide-1 (GLP-1) and its cognate receptor play an important physiological role in maintaining blood glucose homeostasis. A GLP-1 receptor (GLP-1R) polymorphism in which threonine 149 is substituted with a methionine residue has been recently identified in a patient with type 2 diabetes but was not found in non-diabetic control subjects. We have functionally assessed the recombinant GLP-1R variant after transient expression in COS-7 and HEK 293 cells. Compared to the wild type receptor, the variant GLP-1R showed (i) similar expression levels, (ii) 60-and 5-fold reduced binding affinities, respectively, for two GLP-1R full agonists, GLP-1 and exendin-4, and (iii) markedly decreased potencies of these peptides in triggering cAMP-mediated signaling (despite conserved efficacies). In contrast to full agonists, the efficacy of the primary GLP-1 metabolite/GLP-1R partial agonist, GLP-1 (9-36) amide, was essentially abolished by the T149M substitution. By hydropathy analysis, the polymorphism localizes to transmembrane domain 1, suggesting this receptor segment as a novel determinant of agonist affinity/efficacy. These findings reveal that naturally occurring sequence variability of the GLP-1R within the human population can result in substantial loss-of-function. A genetic link between the T149M variant and increased susceptibility to type 2 diabetes remains to be established.

Characterization of a D2-like dopamine receptor (AmDOP3) in honey bee, Apis mellifera

Dopamine is an important neurotransmitter in vertebrate and invertebrate nervous systems and is widely distributed in the brain of the honey bee, Apis mellifera. We report here the functional characterization and cellular localization of the putative dopamine receptor gene, Amdop3, a cDNA clone isolated and identified in previous studies as AmBAR3 (Apis mellifera Biogenic Amine Receptor 3). The Amdop3 cDNA encodes a 694 amino acid protein, AmDOP3. Comparison of AmDOP3 to Drosophila melanogaster sequences indicates that it is orthologous to the D2-like dopamine receptor, DD2R. Using AmDOP3 receptors expressed in HEK293 cells we show that of the endogenous biogenic amines, dopamine is the most potent AmDOP3 agonist, and that activation of AmDOP3 receptors results in down regulation of intracellular levels of cAMP, a property characteristic of D2-like dopamine receptors. In situ hybridization reveals that Amdop3 is widely expressed in the brain but shows a pattern of expression that differs from that of either Amdop1 or Amdop2, both of which encode D1-like dopamine receptors. Nonetheless, overlaps in the distribution of cells expressing Amdop1, Amdop2 and Amdop3 mRNAs suggest the likelihood of D1:D2 receptor interactions in some cells, including subpopulations of mushroom body neurons.

Characterization of a novel D2-like dopamine receptor with a truncated splice variant and a D1-like dopamine receptor unique to invertebrates from Caenorhabditis elegans

We have cloned two novel Caenorhabditis elegans dopamine receptors, DOP-3 and DOP-4. DOP-3 shows high sequence homology with other D2-like dopamine receptors. As a result of alternative splicing, a truncated splice variant of DOP-3, DOP-3nf, was produced. Because of the in-frame insertion of a stop codon in the third intracellular loop, DOP-3nf lacks the sixth and seventh transmembrane domains that are found in the full-length DOP-3 receptor. Reporter gene assay showed that DOP-3 attenuates forskolin-stimulated cAMP formation in response to dopamine stimulation, whereas DOP-3nf does not. When DOP-3 was coexpressed with DOP-3nf, the ability to inhibit forskolin-stimulated cAMP formation was reduced. DOP-4 shows high sequence homology with D1-like dopamine receptors unique to invertebrates, which are distinct from mammalian D1-like dopamine receptors. Reporter gene assay showed that DOP-4 stimulates cAMP accumulation in response to dopamine stimulation. These two receptors provide new opportunities to understand dopaminergic signaling at the molecular level.

Characterization of the 5' regulatory region of the Drosophila Dmdop1 dopamine receptor-gene

The pharmacological and functional properties of many biogenic-amine receptors have been thoroughly investigated. In contrast, knowledge about the transcriptional regulation of receptor genes is limited. Here we describe the structural and functional properties of the promoter region of a dopamine receptor-gene (Dmdop1) from Drosophila. The transcriptional start site was identified by 5'-RACE (5'-rapid amplification of cDNA ends) cloning and primer-extension analysis. A consensus site for transcriptional initiation (INR element) is located 494 bp upstream of the ATG codon of the open reading-frame. The promoter neither contains TATA- nor CAAT boxes but several GC-rich elements. Relative promoter activity was monitored by CAT reporter-gene analysis in different neuronal cell lines. The Dmdop1 promoter contains one activating (-454/+125) and two silencing regions (-1481/-454 and +125/+495). Interestingly, one silencing region harbours a CRE (cAMP responsive element) site. Since the DmDOP1 receptor leads to cAMP production in cells, the CRE site might contribute to the receptors' own expression by cAMP-dependent transcription factors.

Molecular biology of the invertebrate dopamine receptors

Dopamine is found in the nervous systems of both vertebrates and invertebrates. However, the specific actions of dopamine depend on the dopamine receptor type that is expressed in the target cell. As in mammals, different subtypes of dopamine receptors have been cloned and characterized from invertebrates, and these receptor subtypes have different structural and functional properties. Understanding how these receptors respond to dopamine and in which cells each receptor type is expressed is key to our understanding of the role of dopamine signaling. Comparison of the amino acid sequences and experimentally determined functional properties suggest that there are at least three distinct types of dopamine receptors in invertebrates. This review focuses on invertebrate dopamine receptors for which the genes have been isolated and identified, and examines our current knowledge of the functional and structural properties of these receptors, and their pharmacology and expression.

A family of octopamine [corrected] receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster

In invertebrates, the biogenic-amine octopamine is an important physiological regulator. It controls and modulates neuronal development, circadian rhythm, locomotion, 'fight or flight' responses, as well as learning and memory. Octopamine mediates its effects by activation of different GTP-binding protein (G protein)-coupled receptor types, which induce either cAMP production or Ca(2+) release. Here we describe the functional characterization of two genes from Drosophila melanogaster that encode three octopamine receptors. The first gene (Dmoa1) codes for two polypeptides that are generated by alternative splicing. When heterologously expressed, both receptors cause oscillatory increases of the intracellular Ca(2+) concentration in response to applying nanomolar concentrations of octopamine. The second gene (Dmoa2) codes for a receptor that specifically activates adenylate cyclase and causes a rise of intracellular cAMP with an EC(50) of approximately 3 x 10(-8) m octopamine. Tyramine, the precursor of octopamine biosynthesis, activates all three receptors at > or = 100-fold higher concentrations, whereas dopamine and serotonin are non-effective. Developmental expression of Dmoa genes was assessed by RT-PCR. Overlapping but not identical expression patterns were observed for the individual transcripts. The genes characterized in this report encode unique receptors that display signature properties of native octopamine receptors.

Evolution and expression of D2 and D3 dopamine receptor genes in zebrafish

We mined the zebrafish genomic sequence database and identified contigs containing segments of several dopamine receptor genes. By using a polymerase chain reaction amplification strategy, we generated full-length cDNAs encoding a single dopamine D3 receptor and three distinct D2 receptor subtypes. Zebrafish dopamine receptor genes were mapped by using the T51 radiation hybrid panel. The D3 receptor gene (drd3) mapped to linkage group (LG) 24. The three D2 receptor genes were localized to LG 15 (drd2a), LG 16, (drd2b), and LG 5 (drd2c). With the exception of the drd2b gene, each of these map positions was syntenic with regions of human chromosomes containing orthologs of the zebrafish dopamine receptor genes. Whole-mount in situ hybridization was used to investigate expression of the D2 and D3 receptor genes. Expression of the drd3 gene was first detected at mid-somitogenesis and was particularly prominent in somites. Thereafter, the drd3 gene was expressed diffusely throughout the brain and spinal cord. The three D2 receptor genes were expressed throughout the central nervous system (CNS) in distinct but overlapping patterns. In early embryos, the drd2a gene was expressed exclusively in the epiphysis, whereas the drd2c gene was localized to the notochord. After 24 hpf, the drd2a, drd2b, and drd2c genes were differentially expressed throughout the CNS. The identification of dopamine receptor genes in zebrafish should allow us to use the power of zebrafish genetics to analyze the functional properties of this important class of neurotransmitter receptors.

Dopamine receptors in C. elegans

Dopamine regulates various physiological functions in the central nervous system and the periphery. Dysfunction of the dopamine system is implicated in a wide variety of disorders and behaviors including schizophrenia, addiction, and attention-deficit hyperactivity disorder. Medications that modulate dopamine signaling have therapeutic efficacy on the treatment of these disorders. However, the causes of these disorders and the role of dopamine are still unclear. Studying the dopamine system in a model organism, such as Caenorhabditis elegans, allows the genetic analysis in a simple and well-described nervous system, which may provide new insight into the molecular mechanisms of dopamine signaling. In this review, we summarize recent findings on pharmacological and biochemical properties of the C. elegans dopamine receptors and their physiological role in the control of behavior.

The effects of dopamine receptor agonists and antagonists on the secretory rate of cockroach (Periplaneta americana) salivary glands

The acinar salivary glands of the cockroach, Periplaneta americana, are innervated by dopaminergic and serotonergic nerve fibers. Serotonin stimulates the secretion of protein-rich saliva, whereas dopamine causes the production of protein-free saliva. This suggests that dopamine acts selectively on ion-transporting peripheral cells within the acini and the duct cells, and that serotonin acts on the protein-producing central cells of the acini. We have investigated the pharmacology of the dopamine-induced secretory activity of the salivary gland of Periplaneta americana by testing several dopamine receptor agonists and antagonists. The effects of dopamine can be mimicked by the non-selective dopamine receptor agonist 6,7-ADTN and, less effectively, by the vertebrate D1 receptor-selective agonist chloro-APB. The vertebrate D1 receptor-selective agonist SKF 38393 and vertebrate D2 receptor-selective agonist R(-)-TNPA were ineffective. R(+)-Lisuride induces a secretory response with a slower onset and a lower maximal response compared with dopamine-induced secretion. However, lisuride-stimulated glands continue secreting saliva, even after lisuride-washout. Dopamine-induced secretions can be blocked by the vertebrate dopamine receptor antagonists cis(Z)-flupenthixol, chlorpromazine, and S(+)-butaclamol. Our pharmacological data do not unequivocally indicate whether the dopamine receptors on the Periplaneta salivary glands belong to the D1 or D2 subfamily of dopamine receptors, but we can confirm that the pharmacology of invertebrate dopamine receptors is remarkably different from that of their vertebrate counterparts.

Octopamine receptors in the honeybee (Apis mellifera) brain and their disruption by RNA-mediated interference

Octopamine plays important neuromodulatory roles in the honeybee brain. Accordingly, mRNA from a recently identified honeybee octopamine receptor (AmOA1) is distributed throughout the brain. We have evaluated the occurrence of AmOA1 in the antennal lobe (AL) as well as rest of the brain (RB) by western blotting using an antiserum raised against a peptide selected from AmOA1 sequence. In addition to an expected band (78 kDa in the AL), one additional band (72 kDa) was identified from the AL and four bands (48, 60, 72 and 78 kDa) were observed in the RB. These bands were also recognized with antiserum against a different peptide segment from an octopamine receptor ortholog from the fruitfly (OAMB). Significant sequence identity with the peptide segment used to generate the antiserum was only found with OAMB and its splice variants in fruitfly; it was less conserved in other biogenic amine receptors from honeybee and other insects. Furthermore, western blot analysis performed on brains with dsRNA-treated antennal lobes showed a decrease in the intensity of all four bands. This suggests that AmOA1 antiserum specifically recognizes one or more types of AmOA1 receptors in the honeybee brain. We extend our earlier study of RNAi to quantify the rate of spread of dsRNA from a localized injection to other neuropils.

Arthropod 5-HT2 receptors: a neurohormonal receptor in decapod crustaceans that displays agonist independent activity resulting from an evolutionary alteration to the DRY motif

The stomatogastric nervous system (STNS) is a premiere model for studying modulation of motor pattern generation. Whereas the cellular and network responses to monoamines have been particularly well characterized electrophysiologically, the transduction mechanisms that link the different monoaminergic signals to specific intracellular responses are presently unknown in this system. To begin to elucidate monoaminergic signal transduction in pyloric neurons, we used a bioinformatics approach to predict the existence of 18 monoamine receptors in arthropods, 9 of which have been previously cloned in Drosophila and other insects. We then went on to use the two existing insect databases to clone and characterize the 10th putative arthropod receptor from the spiny lobster, Panulirus interruptus. This receptor is most homologous to the 5-HT2 subtype and shows a dose-dependent response to 5-HT but not to any of the other monoamines present in the STNS. Through a series of pharmacological experiments, we demonstrate that this newly described receptor, 5-HT2betaPan, couples with the traditional G(q) pathway when expressed in HEK293 cells, but not to G(s) or G(i/o). Moreover, it is constitutively active, because the highly conserved DRY motif in transmembrane region 3 has evolved into DRF. Site-directed mutagenesis that reverts the motif back to DRY abolishes this agonist-independent activity. We further demonstrate that this receptor most likely participates in the modulation of stomatogastric motor output, because it is found in neurites in the synaptic neuropil of the stomatogastric ganglion as well as in the axon terminals at identified pyloric neuromuscular junctions.

A novel hypothesis on the biochemical role of the Drosophila Yellow protein

In Drosophila melanogaster, the protein product of the yellow gene is necessary for normal pigmentation and male sexual behavior. Although one of the best characterized loci from a genetic standpoint, the function of the Yellow protein in the development of either phenotype is unknown. Here I propose that Yellow acts as a growth factor- or hormone-like molecule in the development of pigmentation and sexual behavior, and discuss the consistency of this theory with experimental observations in flies and humans.

The effects of dopamine agonists and antagonists on the secretory responses in the salivary glands of the locust (Locusta migratoria)

A study has been made on the effect of dopamine on salivary gland secretion rates from isolated locust salivary glands. Application of dopamine induced a concentration-dependent secretion with an IC(50) of approximately 0.3 microM. We investigated the pharmacological profile of this receptor using dopaminergic agonists and antagonists. The effects of dopamine could be mimicked by the selective D1 agonist SKF82958, but not by the D2 agonist TNPA-HCl. The receptor also showed selectively towards certain D1 agonists. SKF82958 was more potent at inducing secretion than SKF81297. We found that dopamine-induced salivary secretions were blocked by the selective D1 antagonist SCH23390, whereas the D2 antagonist sulpiride was relatively ineffective. The cAMP analogue 8-Bromo cAMP also increased secretion rates from isolated salivary glands. These data and the rank order of potency of the agonists and antagonists in this screen suggest that this receptor is a D1-type receptor.

Identification of Drosophila neuropeptide receptors by G protein-coupled receptors-beta-arrestin2 interactions

Activation of G protein-coupled receptors (GPCR) leads to the recruitment of beta-arrestins. By tagging the beta-arrestin molecule with a green fluorescent protein, we can visualize the activation of GPCRs in living cells. We have used this approach to de-orphan and study 11 GPCRs for neuropeptide receptors in Drosophila melanogaster. Here we verify the identities of ligands for several recently de-orphaned receptors, including the receptors for the Drosophila neuropeptides proctolin (CG6986), neuropeptide F (CG1147), corazonin (CG10698), dFMRF-amide (CG2114), and allatostatin C (CG7285 and CG13702). We also de-orphan CG6515 and CG7887 by showing these two suspected tachykinin receptor family members respond specifically to a Drosophila tachykinin neuropeptide. Additionally, the translocation assay was used to de-orphan three Drosophila receptors. We show that CG14484, encoding a receptor related to vertebrate bombesin receptors, responds specifically to allatostatin B. Furthermore, the pair of paralogous receptors CG8985 and CG13803 responds specifically to the FMRF-amide-related peptide dromyosuppressin. To corroborate the findings on orphan receptors obtained by the translocation assay, we show that dromyosuppressin also stimulated GTPgammaS binding and inhibited cAMP by CG8985 and CG13803. Together these observations demonstrate the beta-arrestin-green fluorescent protein translocation assay is an important tool in the repertoire of strategies for ligand identification of novel G protein-coupled receptors.

Developmental changes in expression patterns of two dopamine receptor genes in mushroom bodies of the honeybee,Apis mellifera

The expression patterns of two dopamine receptor genes, Amdop1 and Amdop2, in the developing mushroom bodies of the honeybee brain were determined by using in situ hybridisation. Both genes were expressed throughout pupal development, but their patterns of expression in the three major divisions of mushroom body intrinsic neurons (outer compact cells, noncompact cells, and inner compact cells) were quite distinct. Amdop1 expression could be detected in all three mushroom body cell groups throughout development. Staining for Amdop1 mRNA was particularly intense in newly born Kenyon cells, suggesting that levels of Amdop1 expression are higher in newborn cells than in more mature mushroom body neurons. This was not the case for Amdop2. Amdop2 expression in the mushroom bodies was restricted to inner and outer compact cells during most of pupal development, appearing in noncompact cells only late in metamorphosis or at adult eclosion. In contrast to the case with Amdop1, staining for Amdop2 mRNA was observed in glial cells. Expression of Amdop2 in glial cells was detected only at early stages of glial cell development, when the cells are reported to be actively dividing. This study not only implicates dopamine in the development of honeybee mushroom bodies but also suggests different roles for the two dopamine receptors investigated.

Cloning and characterization of a Caenorhabditis elegans D2-like dopamine receptor

The neurotransmitter dopamine plays an important role in the regulation of behavior in both vertebrates and invertebrates. In mammals, dopamine binds and activates two classes of dopamine receptors, D1-like and D2-like receptors. However, D2-like dopamine receptors in Caenorhabditis elegans have not yet been characterized. We have cloned a cDNA encoding a putative C. elegans D2-like dopamine receptor. The deduced amino acid sequence of the cloned cDNA shows higher sequence similarities to vertebrate D2-like dopamine receptors than to D1-like receptors. Two splice variants that differ in the length of their predicted third intracellular loops were identified. The receptor heterologously expressed in cultured cells showed high affinity binding to [125I]iodo-lysergic acid diethylamide. Dopamine showed the highest affinity for this receptor among several amine neurotransmitters tested. Activation of the heterologously expressed receptor led to the inhibition of cyclic AMP production, confirming that this receptor has the functional property of a D2-like receptor. We have also analyzed the expression pattern of this receptor and found that the receptor is expressed in several neurons including all the dopaminergic neurons in C. elegans.